ELECTRONIC SPECTROSCOPY OF THE AL-H-2 COMPLEX - EXCITED-STATE DYNAMICS AND ORBITAL ALIGNMENT OF THE ALH(A (1)PI) PRODUCT

The weakly bound Al ... H-2 complex was prepared in a pulsed supersoni c beam and studied with laser fluorescence excitation spectroscopy. Tr ansitions to bound bend-stretch levels in the electronic state correla ting with the Al(5s)+H-2 asymptote have been observed. Resonance fluor escence from the excited levels could be detected only very weakly. Th ese excited levels decay mainly nonradiatively, and the excitation spe ctrum was obtained by monitoring emission from the lower Al atomic lev els, 4s and 3d, and AlH A-->X chemiluminescence due to AlH(A (1)Pi) fo rmed in the reactive decay of the excited Al(5s)... H-2 complex. The a ction spectra show a progression of bands which were assigned as a pro gression in the excited state Al-H-2 stretch vibrational mode. The AlH chemiluminescence was recorded with 0.8 nm spectral resolution. Emiss ion in the A-X (0,0) and (1,1) bands, from the upsilon'=0 and 1 vibrat ional levels of the A (1)Pi state, was observed. The relative emission intensity of the Q branches was found to be depressed from that expec ted if the e and f levels of the excited state were equally populated. The experimental spectra were compared with simulations in order to e stimate the rovibrational Lambda-doublet state distribution in the A ( 1)Pi state. A strong propensity to form a levels, with considerable ro tational excitation, was found. The Lambda-doublet propensity is belie ved to be a consequence of the A' symmetry of the Al(5s)+H-2 potential energy surface on which the reagents initially approach one another. (C) 1998 American Institute of Physics. [S0021-9606(98)02544-6].